Alkanes and Alkenes: The Hydrocarbons

Welcome to this comprehensive lesson on alkanes and alkenes for CXC Chemistry. This material covers the entire topic as required by the 2024-2025 CXC syllabus.

Introduction to Hydrocarbons

Hydrocarbons are organic compounds composed solely of hydrogen and carbon atoms. They form the foundation of organic chemistry and are derived primarily from fossil fuels like petroleum and natural gas.

Hydrocarbons are classified based on the types of bonds between carbon atoms:

Saturated hydrocarbons: contain only single bonds (alkanes)
Unsaturated hydrocarbons: contain at least one double or triple bond (alkenes, alkynes)

Alkanes: Saturated Hydrocarbons

Structure and Properties

Alkanes are saturated hydrocarbons with the general formula C_nH_2n+2. They contain only single covalent bonds between carbon atoms.

The Homologous Series of Alkanes

The first ten members of the alkane series are:

Name	Molecular Formula	Structural Formula	Physical State (at room temperature)
Methane	CH₄	CH₄	Gas
Ethane	C₂H₆	CH₃-CH₃	Gas
Propane	C₃H₈	CH₃-CH₂-CH₃	Gas
Butane	C₄H₁₀	CH₃-CH₂-CH₂-CH₃	Gas
Pentane	C₅H₁₂	CH₃-(CH₂)₃-CH₃	Liquid
Hexane	C₆H₁₄	CH₃-(CH₂)₄-CH₃	Liquid
Heptane	C₇H₁₆	CH₃-(CH₂)₅-CH₃	Liquid
Octane	C₈H₁₈	CH₃-(CH₂)₆-CH₃	Liquid
Nonane	C₉H₂₀	CH₃-(CH₂)₇-CH₃	Liquid
Decane	C₁₀H₂₂	CH₃-(CH₂)₈-CH₃	Liquid

Physical Properties of Alkanes

Colorless compounds
Generally odorless (except for commercial products where odors are added for safety)
Less dense than water (they float on water)
Insoluble in water (non-polar molecules)
Melting and boiling points increase with molecular mass
Physical state changes as chain length increases:
- C₁ to C₄: Gases at room temperature
- C₅ to C₁₇: Liquids at room temperature
- C₁₈ and above: Solids at room temperature

Chemical Properties of Alkanes

Alkanes are relatively unreactive compounds, often called "paraffins" (Latin: "little affinity").

1. Combustion

The main chemical reaction of alkanes is combustion (burning in oxygen) to produce carbon dioxide and water:

CH₄ + 2O₂ → CO₂ + 2H₂O + heat

C₂H₆ + 3.5O₂ → 2CO₂ + 3H₂O + heat

General equation: C_nH_2n+2 + [(3n+1)/2]O₂ → nCO₂ + (n+1)H₂O + heat

When insufficient oxygen is available, incomplete combustion occurs, producing carbon monoxide and/or carbon:

2CH₄ + 3O₂ → 2CO + 4H₂O

CH₄ + O₂ → C + 2H₂O

2. Substitution Reactions

Alkanes undergo substitution reactions where hydrogen atoms are replaced by halogen atoms (chlorine, bromine).

Halogenation (in the presence of UV light or heat):

CH₄ + Cl₂ → CH₃Cl + HCl

Further substitution can occur, producing multiple products:

CH₃Cl + Cl₂ → CH₂Cl₂ + HCl

CH₂Cl₂ + Cl₂ → CHCl₃ + HCl

CHCl₃ + Cl₂ → CCl₄ + HCl

Structural Isomerism in Alkanes

Isomers are compounds with the same molecular formula but different structural arrangements. Starting from butane (C₄H₁₀), alkanes can form structural isomers.

Alkenes: Unsaturated Hydrocarbons

Structure and Properties

Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. Their general formula is C_nH_2n.

The Homologous Series of Alkenes

The first few members of the alkene series are:

Name	Molecular Formula	Structural Formula	Physical State (at room temperature)
Ethene (Ethylene)	C₂H₄	CH₂=CH₂	Gas
Propene (Propylene)	C₃H₆	CH₂=CH-CH₃	Gas
Butene	C₄H₈	CH₂=CH-CH₂-CH₃	Gas
Pentene	C₅H₁₀	CH₂=CH-(CH₂)₂-CH₃	Liquid
Hexene	C₆H₁₂	CH₂=CH-(CH₂)₃-CH₃	Liquid

Physical Properties of Alkenes

Similar to alkanes (colorless, odorless, insoluble in water)
Slightly higher boiling points than alkanes with same number of carbon atoms
Lower density than water
Similarly, physical state depends on chain length:
- C₂ to C₄: Gases at room temperature
- C₅ to C₁₇: Liquids at room temperature
- C₁₈ and above: Solids at room temperature

Chemical Properties of Alkenes

Unlike alkanes, alkenes are quite reactive due to the presence of the carbon-carbon double bond.

1. Addition Reactions

The double bond in alkenes makes them undergo addition reactions where the π-bond breaks and new atoms/groups attach to the carbon atoms.

Hydrogenation (addition of hydrogen, requires Ni catalyst):

CH₂=CH₂ + H₂ → CH₃-CH₃

Halogenation (addition of halogens):

CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br

Hydrohalogenation (addition of hydrogen halides):

CH₂=CH₂ + HCl → CH₃-CH₂Cl

Hydration (addition of water, requires H₂SO₄ catalyst):

CH₂=CH₂ + H₂O → CH₃-CH₂OH

2. Markovnikov's Rule

When hydrogen halides (HX) or water (H₂O) add to unsymmetrical alkenes, the hydrogen attaches to the carbon with more hydrogen atoms already attached to it.

CH₃-CH=CH₂ + HBr → CH₃-CHBr-CH₃ (major product)

Not: CH₃-CH₂-CH₂Br

3. Polymerization

Alkenes can undergo addition polymerization to form polymers. This is an industrially important reaction.

n(CH₂=CH₂) → -(CH₂-CH₂)_n-

Ethene → Polyethene (polyethylene)

4. Combustion

Like alkanes, alkenes burn in oxygen to produce carbon dioxide and water:

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O + heat

General equation: C_nH_2n + (3n/2)O₂ → nCO₂ + nH₂O + heat

Isomerism in Alkenes

Alkenes exhibit two main types of isomerism:

1. Structural Isomerism

Different arrangement of carbon skeleton, e.g., but-1-ene and but-2-ene have different positions of the double bond.

2. Geometric (cis-trans) Isomerism

When the same groups are attached to each carbon of the double bond, geometric isomers can form due to the restricted rotation around the double bond.

Comparison Between Alkanes and Alkenes

Property	Alkanes	Alkenes
General formula	C_nH_2n+2	C_nH_2n
Type of bond	Only C-C single bonds	At least one C=C double bond
Saturation	Saturated	Unsaturated
Chemical reactivity	Less reactive	More reactive
Main reaction type	Substitution reactions	Addition reactions
Bromine test	No reaction with bromine solution	Decolorizes bromine solution from red-brown to colorless
KMnO₄ test	No reaction with KMnO₄ solution	Decolorizes KMnO₄ solution from purple to colorless

Industrial Importance

Alkanes

Fuels (natural gas, LPG, gasoline, diesel, kerosene)
Lubricating oils and greases
Waxes for candles and cosmetics
Starting materials for other chemicals

Alkenes

Manufacture of plastics (polyethylene, polypropylene, PVC)
Production of ethanol
Ripening agent for fruits (ethene)
Manufacture of antifreeze (ethylene glycol)
Production of detergents

Practical Tests to Distinguish Alkanes from Alkenes

1. Bromine Test

A solution of bromine in an inert solvent (like tetrachloromethane) is added to the sample.

Alkanes: The red-brown color of bromine persists
Alkenes: The red-brown color quickly disappears as bromine adds across the double bond

2. Potassium Permanganate Test (Baeyer's Test)

A dilute solution of potassium permanganate is added to the sample.

Alkanes: The purple color of KMnO₄ persists
Alkenes: The purple color is discharged as KMnO₄ oxidizes the double bond, forming a diol

Glossary of Terms

Hydrocarbon: A compound consisting only of hydrogen and carbon atoms.
Saturated: Describes molecules containing only single bonds between carbon atoms, with the maximum possible number of hydrogen atoms.
Unsaturated: Describes molecules containing at least one double or triple bond between carbon atoms.
Homologous Series: A series of compounds with the same functional group and similar chemical properties, with each member differing by a -CH₂ group.
Functional Group: An atom or group of atoms that determines the characteristic properties of an organic compound.
Isomerism: The phenomenon where compounds have the same molecular formula but different structural arrangements.
Structural Isomers: Compounds with the same molecular formula but different arrangements of atoms.
Geometric Isomers: Isomers that have the same connectivity but different spatial arrangements due to restricted rotation (e.g., cis-trans isomers).
Addition Reaction: A reaction where atoms or groups are added to a molecule, typically across a multiple bond.
Substitution Reaction: A reaction where one atom or group in a molecule is replaced by another atom or group.
Combustion: A rapid reaction with oxygen that produces heat, light, carbon dioxide, and water.
Polymerization: The process of combining many small molecules (monomers) to form a large molecule (polymer).
Markovnikov's Rule: In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon atom that already has more hydrogen atoms attached.
Halogenation: The introduction of one or more halogen atoms into a molecule.

Self-Assessment Questions

Multiple Choice Questions

What is the general formula for alkanes?
1. C_nH_n
2. C_nH_2n
3. C_nH_2n+2
4. C_nH_2n-2
Which type of reaction is characteristic of alkenes?
1. Substitution
2. Addition
3. Elimination
4. Redox
Butane and 2-methylpropane are examples of:
1. Geometric isomers
2. Structural isomers
3. Functional group isomers
4. Optical isomers
What product is formed when ethene reacts with hydrogen bromide?
1. Ethane
2. Bromoethane
3. Dibromoethane
4. Ethyl bromide
Which test can be used to distinguish between an alkane and an alkene?
1. Litmus test
2. Flame test
3. Bromine water test
4. Benedict's test

Structured Questions

Draw and name the structural isomers of pentane (C₅H₁₂).
Write a balanced equation for the complete combustion of propane.
Describe the mechanism of the addition of bromine to ethene.
Explain why alkanes are relatively unreactive while alkenes are reactive.
Discuss two industrial applications of alkenes and their importance.

Multiple Choice Answers

c) C_nH_2n+2
b) Addition
b) Structural isomers
b) Bromoethane (Note: Ethyl bromide is another name for bromoethane)
c) Bromine water test

Structured Question Answers

The structural isomers of pentane (C₅H₁₂) are:

1. Pentane (n-pentane): CH₃-CH₂-CH₂-CH₂-CH₃

2. 2-Methylbutane (isopentane): CH₃-CH(CH₃)-CH₂-CH₃

3. 2,2-Dimethylpropane (neopentane): C(CH₃)₄
Balanced equation for the complete combustion of propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Mechanism of bromine addition to ethene:

The π-bond in ethene has a high electron density that attracts the slightly positive end of the polarized Br-Br molecule. The mechanism involves:

1. The π electrons attack one of the bromine atoms forming a carbocation intermediate and a bromide ion.

2. The bromide ion then attacks the carbocation to form 1,2-dibromoethane.

This is an electrophilic addition reaction.
Alkanes are relatively unreactive because:

- They contain only strong C-C and C-H single bonds

- These bonds have low polarity

- The electrons are tightly held between atoms

- High energy is required to break these bonds

Alkenes are more reactive because:

- They contain a C=C double bond consisting of a σ-bond and a π-bond

- The π-bond is weaker than the σ-bond

- The π-electrons are more loosely held and are exposed above and below the plane of the molecule

- These π-electrons are readily available for reaction with electron-deficient species
Two industrial applications of alkenes:

1. Polymerization to form plastics:

Ethene is polymerized to form polyethylene (used in plastic bags, bottles, and containers). Propene forms polypropylene (used in ropes, carpets, and plastic components). These polymers are essential in our daily lives and have revolutionized packaging, construction, and manufacturing industries.

2. Production of alcohols:

Ethene reacts with water (in the presence of an acid catalyst) to form ethanol, which is used as a solvent, in alcoholic beverages, and increasingly as a biofuel. This process represents a significant industrial pathway for alcohol production apart from fermentation.